I am running ePO 4.5 with VirusScan Enterprise 8.7i Patch 4 and have set up an On Demand Scan Client Task. It runs every night at 2AM. I am seeing some issues with some systems where the scan runs for a very long time and causes performance issues. When I look at the log file OnDemandScanLog.txt for the systems I often see an issue with encrypted files like this:

Important to note then, is that the on-demand scanner does not have a "scan timeout" feature - it will purposefully run until that scan is complete.

So the first impression from your posting is that you have a file or files that are terribly difficult to scan.

If you want to identify those files, you could use Process Monitor to observe the scan. At a point where the scan appears to be stuck, see what the last file was as recorded by Process Monitor.

Resolutions may be obvious once the file is identified. It may require excluding, or avoiding the scan of archives, or relocating the file and breaking up the scan task into multiple scan tasks so that the troublesome file(s) can be scanned separately.

Another possible outcome is to submit the file in an escalation to McAfee Labs and Support, to see if there's something we can do on our end related to content that might enable faster scan times.